Herein are figures presenting distortion for all three bands as determined
from 8 scans of Stone's deep/dense field "e" on 980113n. First, scans 057 through 064
were reconstructed using the Stone sources in addition to ACT sources as reference stars.
Approximately 2 degrees of each scan overlapped the Stone field. After any residual scan
bias was removed, the Stone source positions were mapped into individual frame coordinates
and differences w.r.t. 2MASS extractions computed. The x-scan (dx) and in-scan (dy)
differences were fitted separately for each band to the following polynomial:
del =c1*x^2 +c2*y*x^2 +c3*x*y +c4*x*y^2 +c5*y^2 +c6*x +c7*y +c8 +c9*x^3 +c10*y^3
Figure 1
plots the average x-scan distortion in J-band as a function of x-scan frame position in the upper-left
panel and as a function of in-scan frame position in the lower-left panel. Note that the units are
pixels. The in-scan
distortion is plotted in the two panels to the right. The same presentation is made
for H-band in Figure 2 and for K-band in
Figure 3. In each plot the solid black lines refer to
the measured distortion and the dotted red lines to the polynomial fit. Note that the
averaged fits track the average observed differences very well in all cases.
The distortion fit reduces the standard deviation of the residuals between 3 and 14 percent,
depending on the band and direction. The improvement is illustrated by before/after histograms
shown in Figure 4 for J-band,
Figure 5 for H-band, and
Figure 6 for K-band. As before, the solid black lines
are without the fit and the dotted red lines are with the fit. When limited to Read1-Read2
extractions only, the improvement is even greater and varies from 5 to 22 percent.
My thanks to Gene Kopan for a set of plots showing a two-dimensional representation of the
measured distortion. Figure 7 breaks the J-band field up into
a 10x10 grid and plots arrows showing the direction and magnitude of the distortion in each
of the 100 grid squares. The vector is plotted at 100 times its actual length.
Figure 8 does the same for H-band and
Figure 9 repeats for K-band.
The polynominal distortion model has been coded and delivered with ver 2.0 of
PosMan, but is currently disabled via namelist flag until it can be incorporated in
ProPhot. This analysis should be repeated in the future to see if there's any significant
change with time or telescope orientation.
http://spider.ipac.caltech.edu/staff/hlm/2mass/distort/distort.html
Comments to: Howard McCallon
Last update: 18 February 1998
